The most accurate response regarding the reactions of the Byzantine and Sasanian Empires to Islamic conquest is:
The Sasanians crumbled due to instability, the Byzantines held out despite losing territory.
The Sasanian Empire faced significant internal challenges and fragmentation, leading to its rapid decline after the Islamic conquests. In contrast, while the Byzantine Empire lost substantial territory, it managed to continue existing for several more centuries, adapting to the new political landscape and occasionally regaining lost lands.